Add function to update string column length in a Dataset

[shaun-actionanalytics]

As a Dataiku platform user, I want to manually set the length of a column in a Dataset scso that I can have a resilient flow.

Dataset settings currently has a host of functions to retireve the dataset's schema and set types, but not length for a string column.

I have flows that fail due to longer string lengths outside fo the sample used to auto detect the schema for a dataset.

This PR adds update_string_column_len to a DSSDatasetSettings class in dataikuapi.dss.dataset.py.

The function takes two inputs:
column - column name as it appears in the schema
length - an integer to set the column length

Conitnued development:
Add error handling to prevent setting the length beyond the maximum set by the connection type
Expand to other column types

I updated the local copy dataiku-api-client-python and was able to successfully update a string column length